Who Discovered the Sun is a Star?

Helios the sun god

Introduction

In ancient Greece during the 7th century BC, scientists like Thales of Miletus (624-546 BC) started to realize that the Sun was not the deity Helios (Apollo) driving his chariot around the Earth, but instead a round fiery ball hanging in space. In 450 BC, Greek philosopher Anaxagoras then became the first person we know of in history to suggest that the stars were actually other suns such as our own, but lying at such great distances that their heat could not be felt here back on Earth. He also (mistakenly) believed that the Sun was a red-hot stone “bigger than the Peloponnese”, but of course, Anaxagoras could not prove his theories, which in any case were considered heretical, and so led to his banishment from Athens.

Two millenia would then pass before Copernicus (1543) produced a fully predictive heliocentric model of the universe, and Galileo (1610), using his newly invented telescope, was able to see Jupiter’s four largest moons, and for the first time show another planet being orbited by its own satellites. Together with his discovery of the phases of Venus, Galileo was able to confirm that the planets were revolving around a rotating sun, and with that discovery, the Earth suddenly went from sitting at the center of the universe, to being a small rock circling a big star.

The precise nature of the stars was still yet to be determined, though, and as the Scientific Revolution (1550-1700) gave way to the Enlightenment (1685-1815), advancements in science and telescopes meant that distances to the stars could be calculated, in the process revealing that their brightness would indeed appear similar to the Sun if not for their vast distant locations. In the mid-19th century, further scientific advancements in spectroscopy and photography, and with them the ability to measure the surface temperature and chemical composition of the Sun and stars, soon provided the definitive proof that the Sun was in fact just another star, after all.

The Sun God

Throughout history, ancient civilizations worshipped the Sun as an all-powerful deity that helped bestow light and life on the cosmos, and amongst the pantheon of ancient gods, the Sun was often seen as the Supreme Deity. In ancient Egypt, for instance, Ra was viewed as the most important god, while in the Vedas of ancient India, Surya was glorified as the creator of the universe and the dispeller of darkness. It is hardly surprising, then, that throughout history those people who sought to relegate the Sun’s position to that of just another star were dealt with harshly.

Ancient Greek Astronomy

Anaxagoras (500-428 BC)

AnaxagorasDuring the 5th century BC, a Greek philosopher named Anaxagoras, who hailed from Asia Minor, came to Athens where he introduced the idea of philosophy, which is the basis for our modern science. He described everything that existed as a mixture of imperishable, infinitely divisible elements, perhaps referring to atoms and molecules, and even postulated as to the possibility of extraterrestrial life.

Anaxagoras also correctly deduced and explained how eclipses occur, said that the Moon was not a luminous body, but rather that it glowed by the reflected light of the Sun, and attempted explanations for meteors, rainbows, and the Sun itself. He postulated that the Sun was merely a stone that had been torn from the Earth and then ignited due to rapid spinning, and that all other heavenly bodies were similarly made of stone. As Anaxagoras was quoted as saying:

“Everything has a natural explanation. The moon is not a god but a great rock and the sun a hot rock.”

His theory may have been inspired by having witnessed a wagon sized meteorite falling from the sky near the Dardanelles in 467 BCE, and having examined the object he concluded that meteorites were pieces of rock that had broken off from the Sun and fallen to Earth. Likewise, this confirmed to him that the stars and the Sun were one and the same—burning rocks, and that the same general rule applied throughout the Universe.

Having fallen foul of Athen’s impiety laws, though, Anaxagoras was sentenced to death, but survived by going into exile. He subsequently retired to the city of Lampsacus, where he taught to a more appreciative and respectful audience until his death in 428 BC.

Aristarchus of Samos (310-230 BCE)

Another Greek mathematician named Aristarchus, credited for being the first person we know of in the Western world to theorize that the Earth revolves around the Sun, referred to as the heliocentric system, also postulated that the stars were in fact distant suns, just like our own. As was noted by Archimedes (287-212 BC) in his book entitled The Sand Reckoner:

“Aristarchus sets the Sun among the fixed stars and holds that the Earth moves round the sun’s circle.”

Aristarchus subsequently attempted to determine the size of the Sun and its distance from Earth, and while there existed no technological equipment for measuring stellar distances at that time, leading to his calculations being significantly inaccurate, he did at least determine that the Sun was much larger object than the Earth. According to his findings, the Sun was around 20 times the Moon’s distance from the Earth, (the actual figure is nearer to 400 times), while the Sun was around 7 times bigger than the Earth, instead of 109 times.

Claudius Ptolemy (100—170 CE)

Claudius Ptolemy of Alexandria, a Greek astronomer renowned in his own era, was convinced of the geocentric (Earth-centered) model of the Universe, and he published his incorrect assertions in his seminal work called Almagest in 150 AD, with the idea then becoming the cornerstone of astronomy for the next 14 centuries until Copernicus published his heliocentric model in 1543. This also included categorizing the Sun as one of the ‘planets‘, a term referring to anything that appeared to wander in the sky, including the Moon, Mercury, Venus, Mars, Jupiter and Saturn; but not the Earth and the “fixed” stars.

The Renaissance Period (1300– 1700 AD)

Copernicus (1473-1543)

CopernicusAround 1400 years later, Copernicus (1473-1543) helped spark the Scientific Revolution by publishing his seminal work ‘De Revolutionibus’ in which he showed that the Earth was just another planet revolving around the Sun. In order to avoid persecution by the Catholic Church, Copernicus did so from his death-bed, and while efforts were made to withdrawn the book from circulation pending suitable corrections being made, there was already beginning to be widespread suspicions voiced across Europe that the Sun was merely a close star.

Giordano Bruno (1548-1600)

In 1584, Italian philosopher and Dominican friar, Giordano Bruno, published two important books in which he propounded the Copernican theory, and argued that if the planets circled the Sun and the Earth was simply another planet, then the Sun should not be considered anything particularly special. As he wrote at the time:

“The composition of our own star and world is the same as that of as many other stars and worlds as we can see.”

In other words, it seemed reasonable to him that the Sun was merely another star, and he subsequently made a distinction between “suns” which generate their own light and heat; and the “earths” and moons which revolve and are nourished and powered by them. One esteemed modern astrophysicist, Steven Soter, has even suggested that Bruno was the first person in history to truly grasp the concept that “stars are other suns with their own planets.”
Unfortunately, The Inquisition found Giordano Bruno guilty of heresy, and he was burned at the stake in 1600, but has since been recognized as a “martyr of science”.

Galileo Galilei (1564-1642)

Galileo and his telescopeIn 1609, Galileo Galilei pointed his recently invented primitive refractor telescope at the stars, which with refinement was eventually capable of magnifying an object by 20 times or more. This proved powerful enough to discover Jupiter’s four moons in 1610, the first objects ever found to be orbiting another planet, and Galileo was then able to use Copernicus’s calculations to show that the planets, including the Earth, did revolve around the Sun. No amount of magnification he could apply to his telescope, however, would allow Galileo to increase and resolve the size of a star into a perceptible “disk”, as stars are simply too far away, thus yielding fewer clues as to their nature. In fact, it would take almost another three centuries before the invention of the spectroscope would prove the precise scientific composition of these stellar bodies, and that the Sun is undoubtedly a star.

Measuring Star Distances

Over the next few centuries, scientist continued working on refining their understanding of the solar system, and measuring distance to other stars.

Stellar ParallaxJohannes Kepler (1571-1630) worked out the laws of planetary motion; while Dutch astronomer Huygens (1629-1695) calculated the distance to the star Sirius. He assumed that stars were the same brightness and that the distance was what determined the appearance of their brightness. As we now know, Sirius is actually 25 times brighter than the Sun, which goes some way to explaining his calculation of Sirius being around 28,000 times the distance between the Sun and the Earth. While this comes to around 0.4 light years, instead of 8.7 light years, it still showed he had an appreciation of the vast distance involved.

Isaac Newton (1642-1727) subsequently discovered his Law of Gravity, which helped explain the reasons behind Kepler’s Laws of Planetary Motion; and German astronomer Friedrich Bessel (1784-1846) became the first person to measure the distance to another star that was not our sun when he calculated 61 Cygni to be 10.3 light-years, a figure very near to the actual value of around 11.4 light-years. Bessel did so in 1838 using a parallax calculation, which involves measuring the distance to a star and comparing its relative angle from a nearer star. Six months later the measurement is repeated when the Earth is on the opposite side of its orbit, which gives a baseline of 186,000,000 miles (299,300,000 kilometers) for an immense triangle. Using the law of sines, it then becomes a very simple matter to calculate the distance to a star.

Astronomical Spectroscopy

In 1666, Isaac Newton showed that a prism separated white light into a spectrum of its constituent parts, rather than creating the rainbow colors that are seen. In 1802, William Wollaston then constructed a spectrometer which showed the Sun’s spectrum on a screen, but noted that there were dark bands of missing colors. In 1814, Joseph von Fraunhofer invented the spectroscope and mapped 574 of these lines, after which a number of scientists helped advance the study of spectroscopy, including Gustav Kirchhoff and Robert Bunsen who in 1857 were able to establish a connection between chemical elements and their own individual spectral patterns.

Astronomical SpectroscopyFurther study revealed that each element absorbs light of a particular color, thus leaving a specific “signature” line, and after spectroscopes were coupled to telescopes, scientist were able to identify additional chemical elements, and work our the chemical composition of the stars, as well as distinguish between nebulae and galaxies in the night sky. During this period, an Italian Jesuit priest and astronomer, Angelo Secchi (1818-1878), became a pioneer in the study of stellar spectroscopy, and through analysis of some 4,000 stellar spectrograms discovered that the stars came in a limited variety of types distinguishable by their unique spectral patterns. He subsequently devised the first stellar classification system, and is recognized as being one of the first scientists to definitively state that the Sun is a star.

We now know that the Sun is a yellow dwarf star composed of around 73% hydrogen, 25% helium, and 2% heavier elements, such as oxygen, carbon, neon, and iron. Its spectral class is GV2, with the G2 indicating a surface temperature of roughly 5505 °C (5778 K); and the V indicating that the Sun is an active star on the main sequence.

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